The Runner’s World Guide to Nutrition and Fueling: Underfueling

The signs that you’re underfueling aren’t always obvious. Your body sends signals that can be difficult to recognize, and you may not even realize that something internally feels off.Every runner should be made aware of signs of underfueling because it can lead to complications with low energy availability (LEA) or relative energy deficiency in sports (RED-S), or even cause further health issues such as injury and illnesses.In The Runner’s World Guide to Nutrition and Fueling, Runner’s World exclusive program led by run coach Mireille Siné and guided by experts, you’ll learn strategies on how to better listen to your body and how to eat according to its caloric needs as an active runner.What You’ll LearnIn The Runner’s World Guide to Nutrition and Fueling, we’ll teach you:How to best identify the symptoms of undernourishmentThe difference between LEA and RED-S, and how both can impact your running and healthWhere “intuitive eating” and running can be useful, and where it may cause symptoms of undernourishment. How to navigate unintentional underfueling vs. intentional underfueling Watch VideoGo DeeperKnowing When You’re UnderfueledUnderfueling can appear differently in every runner, some of the symptoms include:Persistent injuriesIllnessSoreness for extended timeframesSleep difficulty Gastrointestinal problems Mood changesDifficulty concentrating Irregular menstrual cyclesLooking out for these symptoms and being able to recognize the signs of underfueling will vastly improve your running capabilities and ability to recover.LEA and RED-S Low energy availability, or LEA, can occur after a lengthy run where you may not have fueled adequately before, during, or afterward. It is very common for runners to experience, especially following long runs.Luckily, it is easy to dig out of that hole. As long as you fuel properly after the run, and rest and recover well, you’ll be back to normal.However, if you begin consistently underfueling and not taking the proper steps to recover, it can lead to RED-S, or relative energy deficiency in sports. Symptoms of RED-S involve persistent injury, missed menstrual cycles, and unusual amounts of soreness. It’s important to be able to recognize symptoms of both LEA and RED-S to prevent further injury or illnesses due to undernourishment.Intuitive EatingIntuitive eating can be an excellent way of fueling by listening to your body’s own hunger cues and needs. However, in runners, it can lead to unintentional underfueling due to appetite suppression from running. After a run, sometimes your body will not send any hunger cues until a few hours later. Waiting to eat after a run can lead to LEA and even RED-S if it is consistently happening.Unintentional vs. Intentional UnderfuelingUnintentional underfueling can occur after runs when postrun appetite suppression masks hunger signals. Failure to increase nutritional intake as training mileage rises is another way of unintentional underfueling. The more running you do, the higher caloric intake is required, and sometimes that can be tough to address. Runners may not always realize their baseline nutrition is still not enough food to support high mileage. The best way to stay on top of your nutrition as you increase training is to consult with a dietitian or a nutritionist about your habits.Intentional underfueling tends to stem from unhealthy or damaging running stigmas such as “skinny equals fast,” says Amy Goblirsch, RDN. This practice may indicate signs of disordered eating, which according to the Academy of Nutrition and Diabetics, is an umbrella term that describes habits that many practice as a form of weight control.With many conflicting messages out within the running community regarding body type, alongside the rise of weight loss drugs, it’s important to prioritize your well-being. If you recognize any patterns of disordered eating, consult a professional for support.Got questions about performance nutrition?Fueling can be the difference between a run that feels strong and one that falls apart.Through conversations with leading sports dietitians, nutrition experts, and coaches, the Runner’s World Guide to Nutrition and Fueling turns the science of sports nutrition into practical, runner-friendly strategies for everyday training and racing. Available exclusively to Runner’s World+ members on September 10!Casey Healey has been running for 11 years and brings that firsthand experience to the Runner’s World team as a summer 2026 editorial intern. Hailing from Scranton, PA, Casey is now a rising senior at Villanova University, where she plans to graduate with a degree in Communications. She also serves as Co-Sports Editor of her school newspaper, The Villanovan. Outside of running, she enjoys skiing, iced coffee, and walking her mini schnauzer, Winnie.